I picked that one because it does a decent job of showing the men behind the characters or the character of the men playing them.
The first 2 seasons had 3 pertinent episodes to covert/intelligence work although you could argue that it’s all the work of intelligence and much of it is covert. The Blind Banker (S1, E2) had a lovely book cipher, obfuscated by an uncommon Chinese number system, graffitied across (and inside) the buildings of London. A Scandal in Belgravia (S2, E1) had a riddle about a safe code, the puzzle of a locked cell phone, and a short piece of code (not a cipher!) that was very reminiscent of a puzzle someone used to draw me into the manhunt for the vanished Evan Ratliff. The Reichenbach Fall (S2, E3) had a more modern cryptanalysis slant with Moriarty’s hacking macguffin.
At first I refused to watch a Sherlock set in modern times but they’ve done a really wonderful job of infusing it throughout with popular essence and components of canonical Holmes stories without being a dull, dreary reworking of a oh so very English deerstalkered pipe smoker set in the past.
Go watch it if you haven’t seen it and catch up on Season 3 if it’s out by then.